thickness changes in wings pros + cons?

[gouch]

Hi all, I am curious about the reason behind the reason or lack thereof in thinning out the wing as it moves toward the tip.

Some models have it, some are the same thickness all the way through.

Is it done to reduce tip stalling the same way washout is used?
I have a sloper that i made and i'm making a new wing for it
to experiment.
The original i made had about a 50% thinner tip than root. It is a p51 to give some idea of the wing shape.
I'm making a slightly longer span wing to fit same fuse and am wondering should i leave the thickness the same all the way or not.
Any suggestions?

with thanks
Paul

[Ollie]

When the airfoil changes thickness it is important to know if the mean camber line is affected by the airfoil change. A flattening of the mean camber line at the tip introduces aerodynamic washout because, the less cambered airfoil has a less negative zero lift angle of attack. The aerodynamic washout has the same effect as geometric washout in resisting tip stall. The thicker the airfoil the wider its range of low drag but the higher its minimum drag relative to a thinner airfoil of the same family. At model sizes and speeds, thin airfoils generally perform better than thick airfoils because less curvature of the surface stress the flow less and seperation is delayed. The stall characteristics have a lot to do with the leading edge radius and less to do with thickness.

[gouch]

Thanks ollie, I need to be careful asking questions! The answers given generally give me more questions!.
You seem to have no end to your knowledge, where and how did you learn your profession?.

[Ollie]

I'm just an amateur. I learned most of what I know from real professionals like Dr. Mark Drela and Dr. Michael Selig. If you want to know more about airfoils, I suggest you get a copy of X Foil (it's a free down load) and join the X Foil discussion group on Yahoo.

See:
http://raphael.mit.edu/xfoil/
http://groups.yahoo.com/group/xfoil/

[vintage1]

Originally posted by gouch
Hi all, I am curious about the reason behind the reason or lack thereof in thinning out the wing as it moves toward the tip.

Some models have it, some are the same thickness all the way through.

Is it done to reduce tip stalling the same way washout is used?
I have a sloper that i made and i'm making a new wing for it
to experiment.
The original i made had about a 50% thinner tip than root. It is a p51 to give some idea of the wing shape.
I'm making a slightly longer span wing to fit same fuse and am wondering should i leave the thickness the same all the way or not.
Any suggestions?

with thanks
Paul

Paul, I have forgotten what little theory I did know on all this, but a few things may serve to assist in making your decsion.

Fisrtly, teh spitfire with its elliptical wing is suposed to be a swine for tipstalling, but several small versions using thin section (carved from sheet) wings simply don't show this tendencey. Likewise I have a model (in the repair shop at the moment for other reasons) with an elliptical planform, but a constant taper spar, that effectively gives a thinner sing section at the tip. This also does not seem to be in any way more vicious than a constant chord, full depth all the way type wing.

My elliptical wing model seems very slippery as well - which bodes well for a sloper.

I would be striongly tempted to make a wing that does indeed thin towards the tip, and if you are feeling enthusiastic, one that doesn't, and compare the two.

My impression is that the thin tip wing will be better at low speed handling and a bit faster, but not capable of such tight turns at speed.

[Salto]

Ollie,

You have said above: "The stall characteristics have a lot to do with the leading edge radius and less to do with thickness".

How and why is the stall effected by LE radius? Does a sharper radius soften the stall, or the other way around, and why?

I understand your point about thinner sections stressing the flow less. But why then do single surface, highly cambered wings work so well on park flyers and indoor models?

Like Paul, your responses always leave me wanting to know more. I really appreciate your input on this group.

Graham.

[Ollie]

Originally posted by Salto
Ollie,

You have said above: "The stall characteristics have a lot to do with the leading edge radius and less to do with thickness".

Answer:
At high angles of attack before stall, the stagnation point is on the bottom surface just aft of the leading edge and the flow over the top of the wing must first reverse and go around the leading edge. If the flow seperates and doesn't have enough energy the stall is sudden. If the flow does have enough energy after seperation it may reattach forming a laminar seperation bubble on top of the airfoil and just behind the leading edge. An airfoil with a soft stall has seperation starting near the trailing edge which progresses forward as the angle of attack increases.

How and why is the stall effected by LE radius? Does a sharper radius soften the stall, or the other way around, and why?

Answer:
The smaller the leading edge radius, the more energy the flow gives up negotiating the leading edge. The lower the energy of the flow, the earlier its transition to turbulence and the earlier the turbulent flow seperates.

I understand your point about thinner sections stressing the flow less. But why then do single surface, highly cambered wings work so well on park flyers and indoor models?

Answer:
Park fliers and indoor models with highly cambered single surface airfoils have a very limited useful range of angles of attack where coefficient of lift is high and coefficient of drag is low. Those angles of attack are where the angle of upwash in front of the airfoil is close to the the entry angle of the airfoil and the stagnation point is at or very near the leading edge.

Like Paul, your responses always leave me wanting to know more. I really appreciate your input on this group.

Answer:
I always leave you wanting to know more because there IS a lot more to know than I know.

Graham.

[astroboy]

Originally posted by Ollie
I'm just an amateur. I learned most of what I know from real professionals like Dr. Mark Drela and Dr. Michael Selig.


:eek: Ollie, I'm shocked! I figured you must be a retired engineer, at the very least. Well...it doesn't mean I value your opinions any the less.

Jeff

[gouch]

I'm not shocked he said that:)
As I get older and the more I learn about any particular subject, I realise that I don't really know as much as I thought I did. The fact ollie said that he was only an amatuer stamped my initial thoughts on the fact he is indeed far from one, humble yes amatuer no.

I was hoping ollie would give us more of an insight into what he used to do, i'm very interested, same with you sparky paul!

I bet london to a brick you guys could fill many pages on some interesting stories!

Come on guys!:D